1. Field of the Invention
This invention relates semiconductor nanoparticles. More particularly, it relates to the synthesis of photo-luminescent Group III-V quantum dots, such as InP, alloyed with zinc chalcogenides.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Group III-V quantum dots (QDs), such as InP, are a favorable material for commercial applications as an alternative to heavy-metal-containing nanoparticles such as cadmium chalcogenides. Numerous methods to synthesize Group III-V core and Group III-V/ZnX (X=S, Se) core/shell quantum dots have been explored, including hot-injection and single-source precursor methods. Hot-injection involves the rapid injection of precursors at high temperature, triggering nanoparticle nucleation as the reaction temperature drops. This method is generally limited to producing small quantities of QDs because the time required to inject large volumes of precursors is too long for fast nucleation. Hot-injection methods on large scales typically result in an inhomogeneous particle size distribution.
Single-source precursor methods employ a molecular cluster compound containing all of the elements to be incorporated into the nanoparticle material, which decomposes at high temperature to initiate nanoparticle formation. One of the drawbacks of this method, however, is that the nanoparticle stoichiometry is inherently determined by the composition of the cluster compound. Other strategies to synthesize colloidal quantum dots include heating up precursors in solution, with the addition of other reagents during the course of the reaction.
In U.S. Pat. No. 7,588,828 (issued Sep. 15, 2009, the entire contents of which are incorporated herein by reference) entitled “Preparation of Nanoparticle Material,” we disclosed a scalable “molecular seeding” method to synthesize Group III-V semiconductors using a Group II-VI molecular cluster as a template for nanoparticle growth. The molecular cluster could be formed prior to addition to the reaction flask, or formed from suitable reagents in situ. In one example, the zinc cluster, [Zn10S4(S(C6H5))16][NH(C2H5)3]4, was used as a source of ZnS “seeds” to act as templates for the growth of InP nanoparticles.
A challenge in the synthesis of InP quantum dots is to achieve emission in the blue region of the visible spectrum. For the band gap of InP to be sufficiently large to emit blue light, a very small particle size is required. At this size, InP particles are intermediate between clusters and nanoparticles. As a result, the particles are highly unstable. Since the bulk band gap of ZnS is much wider than that of InP, by alloying ZnS into the InP cores their physical dimensions can be retained while blue-shifting their absorption and emission. Thus, strategies to increase the amount of Zn alloyed into the InP core are favorable for applications where enhanced blue light absorption and emission is of benefit, such as quantum dot light-emitting diodes.
Several examples of InP quantum dot synthesis without employing a molecular seeding cluster have been documented in the prior art. T. Kim et al. report the synthesis of InP—ZnS alloyed quantum dot cores using indium acetate, zinc acetate, dodecanethiol (DDT) and palmitic acid (CH3(CH2)14COOH) in 1-octadecene as the non-coordinating reaction solvent [T. Kim et al., J. Phys. Chem. Lett., 2012, 3, 214]. In a typical synthesis, In(OAc)3, Zn(OAc)2 and palmitic acid were mixed in a 1:3:3 molar ratio with DDT (0-3 molar equivalents) and 1-octadecene (ODE). After degassing, the mixture was heated to 210° C. A 1-mmol solution of (TMS)3P in ODE was added drop wise, at a rate of 1 mL·h−1, over 5 hours. It was reported that the optical properties of the nanoparticles could be controlled by manipulating the relative ratio of DDT and palmitic acid surfactants added to the reaction flask. An increase in the quantity of DDT led to a blue-shift in PLmax, suggesting a higher ZnS content. All wavelengths emitted with a quantum yield<1%, which could be increased to between 20-45% by shelling with ZnS. Shelling was achieved by the addition of Zn(OAc)2 to the solution of cores at room temperature, then heating at 230° C. for 5 hours, after which DDT was optionally added before further annealing for 2 hours (depending on the required PLmax). However, the resultant nanoparticles displayed poorly defined UV-visible absorption spectra, suggestive of broad particle size distributions.
Researchers at CEA, Grenoble, have reported a single-step procedure to synthesize InP/ZnS nanoparticles [L. Li & P. Reiss, J. Am. Chem. Soc., 2008, 130, 11588]. 0.1 mmol of each of indium myristate (In(MA)x), zinc stearate, (TMS)3P and DDT were mixed with 8 mL of ODE at room temperature. The reagents were then heated to 230-300° C. at a rate of 2° C. s−1 and held for a fixed time (between 5 minutes and 2 hours). By varying the reaction conditions, quantum yields in the range of 50-70% and FWHM values between 40-60 nm could be attained. When the amount of Zn and S precursors were reduced (In:P:MA:Zn:DDT=1:1:3.5:0.3:0.3), larger particles were synthesized with 68% quantum yield and a narrow size distribution, however the photo-stability was reduced with a T50 (time after which the quantum yield falls to 50% of its original value) of 15 hours under UV irradiation. In a later publication, XPS analysis revealed that the particles grown by this procedure have a homogeneously alloyed InPZnS structure comprising In—P, Sx—In—P1-x, and In—S components [K. Huang et al., ACS Nano, 2010, 4, 4799]. In a further publication [U. T. D. Thuy et al., Appl. Phys. Lett., 2010, 97, 193104], the effect of zinc stearate concentration was studied. Increasing the amount of zinc in the reaction solution resulted in larger particles emitting at longer wavelengths.
A variation on the method described by Li and Reiss, without the addition of DDT, was used to synthesize InP cores that were subsequently shelled with a compositionally graded ZnSeS alloy [J. Lim et al., Chem. Mater., 2011, 23, 4459]. Quantum yields in the region of 45% were achieved for the shelled nanoparticles.
The method disclosed by CEA, though a fairly rapid reaction producing reasonable quantum yields, has a number of disadvantages. Firstly, reaction temperatures up to 300° C. are required and the heating rate of 2° C.·s−1 is unfeasible on a large scale, suggesting that the reaction would not be easily scalable. In addition, high quantum yield comes at the expense of stability.
Xu et al. report a rapid, single-step, single-pot method to synthesize InP and InP/ZnS nanoparticles with quantum yields in the region of 30% and 60%, respectively, exhibiting FWHM values around 60 nm [S. Xu et al., J. Mater. Chem., 2008, 18, 2653]. In a typical synthesis, 0.1 mmol of InCl3, 0.1 mmol of stearic acid, 0.1 mmol of zinc undecylenate, 0.2 mmol of hexadecylamine (HDA), and 3 mL of ODE or methyl myristate were mixed. The flask was purged with nitrogen and heated to 280° C. under vigorous stirring. 0.5 mL of a 0.2 mmol·mL−1 solution of (TMS)3P in ODE was injected in quickly, then the solution was held at 240° C. for 20 minutes. The particle size could be manipulated by altering the concentrations of the zinc undecylenate and HDA. The PL could be tuned from 480-750 nm, but to achieve longer emission multiple injections were required. To shell the cores, the reaction solution was cooled to room temperature, then 0.15 mmol of zinc dithiocarbamate and 2 mL of ODE were added. After purging with nitrogen, the solution was heated to 230° C. for 20 minutes. The shelled nanoparticles were reported to display good stability against photo-bleaching. The method described by Xu et al. yields fairly bright nanoparticles with extensive wavelength tuneability, however the synthesis requires rapid injection, which is difficult to perform on a large scale.
In a variation of the method described by Xu et al., InP/GaP/ZnS quantum dots were synthesized by shelling a ZnInP core with GaP and ZnS [S. Kim et al., J. Am. Chem. Soc., 2012, 134, 3804]. Different precursors were used, however, thiol was again eliminated from the core synthesis. In a typical reaction 0.12 mmol of In(OAc)3, 0.06 mmol of Zn(OAc)2, 0.36 mmol of palmitic acid and 8 mL of ODE were degassed at 110° C. for 2 hours. Under an inert atmosphere, the solution was heated to 300° C. and a solution of 0.06 mmol of (TMS)3P in 1 mL of ODE was injected in quickly. The reaction solution was then held at 230° C. for 2 hours, yielding cores with PLmax=530 nm. The cores were shelled with GaP and ZnS. The GaP layer was incorporated to mitigate the lattice mismatch between InP and ZnS. Cores emitting at 590 nm were also shelled with GaP and ZnS, causing the emission to shift to 615 nm with a quantum yield of 58%.
The method described by S. Kim et al. utilizes hot-injection of (TMS)3P, which is difficult to replicate on a large scale. Further, their quantum yield for shelled red nanoparticles was quite low.
In summary, the methods described in the prior art fail to encompass all of the criteria to produce photo-luminescent Group III-V quantum dots on a large scale with optical properties that are suitable for commercial applications.